Scientists Look to Clouds for Better Climate Forecasts

This image of clouds over the southern Indian Ocean was acquired on July 23, 2007 by one of the backward (northward)-viewing cameras of the Multi-angle Imaging SpectroRadiometer (MISR) instrument on NASA's polar-orbiting Terra spacecraft.

Research scientists meeting this month in Key West, Fla., literally have their heads in the clouds. More than 400 scientists, engineers, theoreticians and pilots from NASA, other government agencies, academia and industry are conducting an experiment to obtain better predictions of global climate change by examining how cirrus clouds form and evolve.

"This is a very ambitious attempt to better understand what makes cirrus clouds tick," said Dr. Randy Friedl, a research scientist from NASA's Jet Propulsion Laboratory, Pasadena, Calif. "We are addressing key uncertainties in how these clouds affect climate change."

Friedl is lead scientist on NASA's WB-57F aircraft, one of six aircraft being used in the experiment. He is one of several JPL research scientists and teams participating in the effort, called the Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment.

Cirrus clouds, or "ice clouds," are high, cold clouds composed of microscopic ice crystals that range in size from a few to thousands of micrometers. (A human hair is about 1,000 micrometers thick.) In the tropics, these clouds form at altitudes from about 6,000 meters (20,000 feet) to 18,288 meters (60,000 feet). Both the characteristics of tropical cirrus clouds and the way they interact with solar and infrared radiation may help determine surface temperatures on Earth. Scientists are also looking at the effects that water vapor traveling in the tropical tropopause, located between the stratosphere and the troposphere, may have on Earth's climate.

JPL scientists are involved principally with instruments on NASA's WB-57F aircraft, ER-2 aircraft and Terra satellite. The planes will be used for remote and direct sampling of atmospheric conditions and characteristics of cirrus clouds, particularly in the tropopause region.

One of the myriad of instruments that scientists will use to make these observations is the JPL Microwave Temperature Profiler. This instrument measures a vertical profile of atmospheric temperatures within 6 kilometers of flight level (about 4 miles above and below the aircraft), enabling pilots and scientists to find the tropopause region. It can also detect the presence of gravity waves, which are excited by thunderstorms; these waves eventually break up and dissipate energy, which heats up the atmosphere.

"It is important to understand the impact of the cirrus outflow from thunderstorms on Earth's radiation budget (the amount of radiation that enters and leaves Earth)," said Dr. M.J. Mahoney, principal investigator of the JPL Microwave Temperature Profiler. "Temperature is a fundamental parameter in understanding virtually all atmospheric phenomena."

Other instruments being used in the experiment are the JPL Near-Infrared Laser Hygrometer, which will measure water vapor while flying on both planes through cirrus clouds, and the JPL-developed Aircraft Laser Infrared Absorption Spectrometer. The spectrometer will use new-technology lasers to make first-ever, direct measurements of isotopic water vapor. "This will give scientists a whole new handle on understanding how water moves from the troposphere to the stratosphere," said Dr. Chris Webster, JPL principal investigator of the Aircraft Laser Infrared Absorption Spectrometer.

Also on the WB-57 is the Solar Spectral Radiometer. Designed and built by JPL's Atmospheric Chemistry Division and Microdevices Group, it measures total direct sunlight as well as small-scale variability in radiation reflected from clouds. Using detectors developed at JPL that are faster and more sensitive than those in similar instruments, the Solar Spectral Radiometer will help scientists develop better models of cirrus clouds.

JPL research scientist Dr. Ralph Kahn and a team working with the Multi-angle Imaging SpectroRadiometer, flying on NASA's Terra spacecraft, will be investigating the properties of tiny particles called aerosols that are suspended in the air. Sources of aerosols include natural activity on Earth such as volcanic eruptions and forest fires, as well as man-made pollution. In addition to reflecting and absorbing sunlight, these particles are believed to exert an important, indirect effect on climate by changing the properties of clouds. The team will analyze satellite measurements with those taken simultaneously from the aircraft to see how well their instrument can observe aerosols from space in the presence of thin cirrus clouds. They will also attempt to retrieve properties of those clouds using the same approach that works for smaller atmospheric aerosol particles.

Participants in the Cirrus Regional Study of Tropical Anvils and Cirrus Layers - Florida Area Cirrus Experiment include researchers fromJPL; Ames Research Center, Moffet Field, Calif.; Goddard Space Flight Center, Greenbelt, Md.; and Langley Research Center, Hampton, Va. Other participants are from the National Oceanic and Atmospheric Administration, Washington, D.C.; the National Center for Atmospheric Research, Boulder, Colo.; and various universities and companies. JPL is managed for NASA by the California Institute of Technology in Pasadena.